Pumice is a naturally occurring volcanic product. Due to the manner in which it is created in nature it has a soft texture and is rather porous.
Its ability to withhold and release fertilizer slowly is quite high and therefore when used in a growing medium less frequent fertilizing is required. Pumice is also water retentive but drains exceptionally well and therefore it would be difficult to over water and create conditions conducive to root rot.
Root are not able to penetrate pumice, as is also the case with other media like Leca. However unlike Leca where the surface of the particle is hard, pumice is soft and based on internet surveys, it becomes apparent that roots like pumice and readily ramify in mixes which contain a fair amount of pumice in it.
If I had only one soil component to use for bonsai it would be 100% pumice. If you have akadama, add some. - Michael Hagedorn
Due to the very good drainage qualities of pumice it is very often used to assist a sick tree to regain vigor and health. In such a case 100% pumice may be used.
Here are a few recipes from leading international bonsai practitioners:
80% pumice and 20% composted pine bark for strong roots and early tree development. Pumice provides the widest range of healthy growing conditions. You can over water it, under-water it, and it creates great fine root growth second only to pumice mixed with about 50% akadama.
1 part pumice, 1 part lava, 1 part akadama, 1 handful decomposed granite and 1 handful horticultural charcoal.
Akadama, lava rock, and pumice. Use a 1:1:1 ratio respectively for conifers and a 2:1:1 ratio respectively for deciduous and broadleaf evergreen species.
2 parts akadama, 1 part hyuga (Japanese pumice) and 1 part coarse river sand.
Pumice (Hyuga), Japanese brand
Suited to mame, shohin and general bonsai growing mixes. Available in 2L bag, 5L bag and 16L bag.
Suitable for medium to large bonsai and as drainage layers. Available in 2L bag, 5L bag and 16L bag.